Man perhaps erected Helsinki houses (4)
I believe the answer is:
isle
'man perhaps' is the definition.
(Isle of Man is an example)
'erected helsinki houses' is the wordplay.
'erected' is a reversal indicator (in a down clue, letters go up).
'houses' indicates the answer is hidden within the clue (part of the clue houses the answer).
'elsi' can be found hidden inside 'helsinki'.
'elsi' reversed gives 'ISLE'.
(Other definitions for isle that I've seen before include "Small region of land" , "I shall hear" , "Perspire" , "Small seabound place" , "Man conceivably" .)